Polysaccharides contain structured and even crystalline portions which make them less soluble in water and also difficult to break down to their recurring units to obtain the underlying monomeric units. In the case of cellulose, these monomeric units are glucose units which can be converted to useful compounds, including ethanol or similar alcohols obtained through fermentation.
Ethanol and other chemical fermentation products typically have been produced from sugars derived from high value feedstocks which are typically high in starches and sugars, such as corn. These high value feedstocks also have high value as food or feed.
It has long been a goal of chemical researchers to improve the efficiency of depolymerizing polysaccharides to obtain monomeric and/or oligomeric sugar units that make up the polysaccharide repeating units, it is desirable to increase the rate of reaction to yield free monomer and/or oligomers units in order to increase the amount of alcohol that may be obtained by fermentation of the monomeric and/or oligomeric units.
Much research effort has been directed toward enzymes for depolymerizing polysaccharides, especially to obtain fermentable sugars which can be converted by fermentation to target chemicals such as alcohols.
However, some polysaccharides, such as cellulose, are relatively resistant to depolymerization due to their rigid, tightly bound crystalline chains. Thus the rate of hydrolysis reaction to yield monomer may be insufficient for use of these polysaccharides in general and cellulose in particular as a source for saccharide monomers in commercial processes. Enzymatic hydrolysis and fermentation in particular can also take much longer for such polysaccharides. This in turn adversely affects the yield and the cost of fermentation products produced using polysaccharides as substrates
A number of methods have been developed to weaken the ordered regions of polysaccharides to obtain more efficient monomer release. Most of these methods involve pre-treatment of the polysaccharide prior to reactions to obtain monomers. Pretreatments chemically and/or physically help to overcome resistance to enzymatic hydrolysis and are used to enhance cellulase action. Physical pretreatments for plant lignocellulosics include size reduction, steam explosion, irradiation, cryomilling, and freeze explosion. Chemical pretreatments include dilute acid hydrolysis, buffered solvent pumping, alkali or alkali/H2O2 delignification, solvents, ammonia, and microbial or enzymatic methods.
These methods include acid hydrolysis, described in U.S. Pat. No. 5,918,780 to Foody, et al. The referenced patent also describes the deficiency of acid hydrolysis and teaches use of pretreatment and treatments by enzymatic hydrolysis.
U.S. Pat. No. 5,846,787 to Ladisch, et al. describes enzymatically hydrolyzing a pretreated cellulosic material in the presence of a cellulase enzyme where the pretreatment consists of heating the cellulosic material in water.
In US Patent Application No. 20070031918 A1, a biomass is pretreated using a low concentration of aqueous ammonia at high biomass concentration. The pretreated biomass is further hydrolyzed with saccharification enzymes wherein fermentable sugars released by saccharification may be utilized for the production of target chemicals by fermentation.
Zhao, et. al. (Zhao, Y. Wang, Y, Zhu, J. Y., Ragauskas, A., Deng, Y. in Biotechnology and Bioengineering (2008) 99(6) (1320-1328)) have shown that high levels of urea, when combined with sodium hydroxide as a means of swelling the cellulosic matrix, improves the accessibility of the isolated cellulose for subsequent enzymatic hydrolysis. This may be attributed to the effect of the urea in disrupting the hydrogen bonding structures that are important in producing the more ordered regions of the polysaccharide.
Borsa, et al. (J. Borsa, I. Tanczos and I. Rusznak, “Acid Hydrolysis of Carboxymethylcellulose of Low Degree of Substitution”, Colloid & Polymer Science, 268:849-657 (1990)) has shown that introduction of very low levels of carboxymethylation accelerates the initial rate of hydrolysis when cellulose is subjected to acid hydrolysis.
The process taught in Borsa et al. treats cotton fabrics by dipping in caustic and then sodium chloroacetate solution resulting in mild surface substitution at levels below 0.1 D.S. This is illustrated in FIG. 1 of Borsa et al. which shows a maximum D.S. of about 95 millimoles per basemole after 20 minutes of carboxymethlation, or 0.095 D.S. if using the numbering for D.S. as carboxymethyl groups per anhydroglucose unit.
Borsa et al. used a large excess sodium hydroxide (of mercerizing strength) but a small amount of chloroacetic acid. Further, reported yields in Borsa, et al. of hydrolyzate are on the order of 0 to 35 milligrams per gram, or not more than 3.5% while the untreated cotton control yields about 2.5% hydrolysis under the same conditions.
In U.S. Pat. No. 6,602,994 to Cash, et al., it has been shown that low levels of cellulosic derivatization aids in reducing the amount of mechanical energy required for defibrillation. Cellulose is first swelled with alkali and then reacted with chloroacetic acid or other suitable reagents to obtain derivatized cellulose.